replacing Cast Iron Stack with Pvc to Clay Sewer pipe

Hello Everyone,
I'm in the middle of a DIY nightmare, remodeling a second floor bathroom for my mom. The house is 90 or so years old, with some really bad renovations and additions done by the previous owner, about 20 years ago.
I don't want to go into the whole story but basically the DWV for the two bathrooms were hacked up by the previous owner so I planned to just remove the CI down to the basement.
Unfortunately the CI came loose at the basement floor. it basically just pulled right out by hand to reveal the 4"CI stack was cut just below the hub and then dropped into a 6"clay sewer branch line. Can I use an o ring? or do I have to cut up the concrete and do the coupling over the clay tile?
Seems to me the o ring or donut would be a tighter more secure fit.
Is there a temporary solution that can hold a couple weeks so I can finish the project and go back to cutting up the basement floor?

John, Thank you for the quick reply. I had seen those, and ran around the city looking for one at the big box stores with no luck. Would this be a good permanaent solution? Is there anything I should do to finish over it?
Here is what I'm dealing with.

I was able to get the o ring from a plumbing supply 5 minutes from me yesterday. Got it home and installed it but was loose in the clay pipe. Turns out the clay pipe is apparently cracked vertically and looks like it had shifted underground at some point. If inserted into the clay bell squarely, the new waste pipe is about 15-20 degrees off from vertical. Only reason the old 4" CI stack was vertical is because someone had previously just dropped the 4" CI straight into the 6" clay and just cemented over it. Not sure if that is the way it used to be done or if it's a hack job. Could also have been leaking for a while because we have had water pool up around the old stack but thought it was coming from the foundation wall and just pooling there as a low point.
I'm thinking now I have to take that cracked clay pipe stub out and fit the 6x4 coupling over the next pipe? Can I get away with just chipping out enough concrete to get out the next piece or do I have to excavate the whole thing?

Its best to dig enough till you get to the horizontal pipe, make a clean cut (chain snap cutters works best), then install a 4" x 6" no sheer coupling, then a straight piece of cast iron with a hub, then a 90Âº pointing you up where your original stack was, finally enough cast iron to get you above the floor. Can not use PVC under the slab in Chicago, and depending on the size of your home (how many stories and how many family building) you might not be able to use PVC for the piping above grade either. You can use PVC above grade if the building is less than three stories and or less than a three family home.